An Experimental Investigation on Rotor-to-Rotor Interactions of Small UAV Propellers

Abstract

In the present study, an experimental investigation was performed to study the effects of rotor-to-rotor interactions on the aerodynamic and aeroacoustic performances of small UAVs. It was found that, while the thrust coefficients of rotor were independent of the separation distance, the thrust fluctuations were found to increase dramatically as the separation distance decreased. An enhancement of ~ 250% was confirmed for the twin-rotor case (i.e., L= 0.05D) in comparison with that of the single rotor case, which is believed to be caused by the complex flow interactions within rotors as revealed by the detailed PIV and Stereoscopic PIV measurements. Reducing the separation distance not only intensified the force fluctuations, but also increased the aeroacoustic noise level of the baseline case. Comparing to the L = 1.0D case, a maximum enhancement of ~3 dB in aeroacoustic noise was recorded for the L = 0.05D case, which is caused by the severely thrust fluctuations and turbulent flow interactions within rotors.

Modeling Multirotor Aerodynamic Interactions Through the Vortex Particle Method

“…6 (left), where features of the topology can be clearly identified: a well-defined structure in the near field (z < 0.5D), an onset of leapfrogging at z ≈ 0.5D, and a transition into turbulent breakdown at z ≈ 1.0D. The simulation converges to a mean C T of 0.1013 and a steady fluctuation (standard deviation) of 0.001, meanwhile Zhou et al 18 and Ning 19 report an experimental mean C T of 0.1007 and a fluctuation of 0.008. This results in an error of only 0.5% between the simulation and experimental mean C T , meanwhile the fluctuation is largely underpredicted.…”

“…In contrast, distributed propulsion eVTOL aircraft are typically wing-borne during cruise, and the axes of rotation of the rotors are roughly aligned with the direction of flight, with the velocity of the aircraft adding directly to the axial inflow and advance ratio of the rotors. Even though previous studies have shown that multirotor interactions could drop the thrust performance by only a small amount [15][16][17]20 (∼4% relative to the isolated rotor), they also show that noise could be increased by as much as 3 dB, 18,19 corresponding to an acoustic signature two times more intense, which is a critical factor to consider in the design of urban mobility aircraft. Experimental results indicate that this increase in noise is directly related to unsteady loading during aerodynamic interactions.…”

“…In order to simulate both hovering and forward-flight multirotor configurations at both low and high Reynolds numbers, two different rotor geometries are used. The hovering case uses a quadcopter rotor matching the geometry of the DJI Phantom II rotor described by Zhou et al 18 and Ning, 19 and the low-Reynolds simulation of side-by-side hovering rotors is compared to reported particle image velocimetry (PIV) and load cell measurements. This rotor is 240 mm in diameter, and its geometry approximates the optimum rotor design obtained by imposing a constant effective angle of attack along the blade.…”

“…Prior to multirotor modeling, the propeller model is first validated in a single-rotor configuration at both low and high Reynolds number. In this section the single rotor is simulated in both hover and forward flight, and results are compared to experimental measurements reported by Zhou et al, 18 Ning, 19 and McCrink and Gregory. 52…”

“…Coaxial and tandem rotor interactions have been studied for decades in the rotorcraft community, meanwhile the side-by-side multirotor configuration has remained relatively unexplored. Recent experimental works [16][17][18][19][20] have studied the counter-rotating configuration where neighboring rotors spin in opposing directions (clockwise versus counterclockwise), however, no attention has been given to the co-rotating configuration where neighboring rotors spin in the same direction (both clockwise, or both counterclockwise). Also, these studies only considered the hovering case, meanwhile the effects of multirotor interactions in forward flight remain unexplored.…”